A requirement has long existed in industry to test multiple conductor cables and harnesses for functionality. Three basic electrical tests are conducted routinely, the continuity test, the short circuit test and the insulation test. The insulation test is normally conducted by the manufacturer of the Cable.
Relatively simple cables may be manually tested using an ohmmeter to measure circuit path resistance. However this technique is not practical with cables having a large number of conductors. Since any conductor could short to any other conductor in a cable, the number of potential shorted paths in a cable with n conductors approaches ((n*(n-1))/2)-n. For a 100 conductor cable a complete check for short circuits approaches 4,850 tests and hence it is not practical to manually perform the test.
Presently known Cable Testers automatically test cables and harness assemblies. A typical cable testing system employs some means for mating the connectors of the cable to the inputs and outputs of the test equipment. Within the test equipment, two groups of switches connect a conductor at each end of the circuit path under test to a resistance measurement circuit. The resistance measured is compared to the expected resistance for that path to determine a pass/fail condition.
A limitation in the currently known Cable Testing Systems is that they use relay switches to perform the switching function. Relays are expensive, bulky and slow. Since a large number of switch contacts are needed (200 for the example above with a 100 conductor straight through cable) a relay switching implementation would cost thousands of dollars and would be of considerable size and weight. The inherent unreliability of an electromechanical device and the slowness of the relay also cause a delay in the switching function. Upon activation or deactivation of the switches the switch contacts of a relay may bounce. A reliable switch open or closed state will not occur for between 50-150 mSec after relay activation. This limits the rate at which circuits may be tested at a rate of approximately 10 circuits per second.